This invention relates to a thermal sensing circuit and, in particular, to a thermal sensing circuit, using few components, for producing a relatively large output signal for small changes in temperature.
Thermal sensors are needed in a multitude of applications, such as sensing when or whether a critical temperature has been exceeded or whether a malfunction has occurred. By way of example, when a malfunction such as a short circuit occurs on or about a circuit, excessive current flows and the power dissipation increases, resulting in the rise of temperature on or about the circuit. When a malfunction has been detected or a critical temperature exceeded, it is generally necessary to respond quickly.
A problem with prior art thermal sensing circuits is that they rely on the change in one parameter to control the turn on or turn off of a controlled device. As a result large changes in temperature must occur to obtain a noticeable response, if additional amplification is not introduced.
This is best explained with reference to FIG. 1A which shows that a portion of a band gap voltage (KV.sub.BG) is applied between the base and emitter of a bipolar transistor Q1, also referred to herein as the control led device. Generally, the band gap voltage, KV.sub.BG, applied to the base of Q1 is held at a relatively fixed value as a function of temperature, as shown in FIG. 1B. Temperature sensing is achieved by relying on the well known principle that the base-to-emitter voltage (V.sub.BE) Of a bipolar transistor decreases at a predetermined rate (e.g., -2 millivolts per degree centigrade for low level currents), as shown in FIG. 1B. Therefore, in operation, the collector current through Q1 increases with temperature as a function of the difference between the fixed KV.sub.BG and the decreasing value of the V.sub.BE of Q1. However, the increase in the collector current is relatively slow and shallow. Note that the "effective" input signal is the difference between the base voltage (KV.sub.BG) and the V.sub.BE of Q1 which is decreasing at the predetermined rate shown in FIG. 1B. Assuming 75.degree. C. to be a critical point at which a response is desired, there must be a substantial increase in temperature above the critical temperature before Q1 saturates and causes the voltage at its collector to go to, or close to, ground potential.
FIG. 1A illustrates the application of a fixed reference voltage KV.sub.BG to the base of a transistor, Q1. Assume, for example, that KV.sub.BG is equal to 500 mv. Assume further that the V.sub.BE of Q1 is 600 mv at 25.degree. C. and that its V.sub.BE decreases at the rate of approximately -2 mv/.degree.C. Thus, when the temperature reaches 75.degree. C., the V.sub.BE of Q1 is equal to 500 mv and Q1 is ready to start conducting. The relationship between KV.sub.BG and the V.sub.BE of Q1 is shown in FIG. 1B. As the temperature increases above 75.degree. C., Q1 starts conducting but it will not be turned-on hard until the temperature reaches a substantially higher value. Assuming that the V.sub.BE of Q1 decreases by -2 millivolts per degree centigrade and that the collector current of Q1 increases by 4 per cent for each millivolt of added bias to the base voltage, there must be a very large temperature change to effectuate a significant change in the collector current. Therefore, a problem exists when it is desired to have significant current flow for small temperature change above the critical values.
Admittedly, a greater change in collector current for a smaller change in temperature may be obtained by the insertion of amplifiers at the output (collector) of Q1. However, there are many systems in which it is undesirable to introduce the use of amplifiers either because of cost consideration and/or where the reduction in the number of components is of importance.
Thus, it is desirable and/or necessary to have a simple thermal sensing system in which relatively large currents (or voltages) can be produced for small changes in temperature about a critical temperature.